A tracking device like a computer mouse can use an optical movement sensor to determine motion of the mouse. One type of optical movement sensor operates by focusing and sensing images of a surface on which the mouse moves. The surface is illuminated by, for example, light emitting diodes (LEDs) or a laser to emphasize imperfections on the surface.
The mouse determines movement by periodically sampling images from the surface and comparing new images to previous images. A process of image correlation is then used to extract and compute a composite offset of the new image as compared to the previous image. The image correlation process can produce inaccurate results when the image is out of focus. Blurring of the image reduces edge contrast and the reliability of the computed composite offset. Loss of focus limits the range of acceptable surfaces on which the image correlation process will function.
Small imperfections on a lens in the optical path can accentuate geometric variations of the optical path. Geometric variations can cause loss of focus and degrade the performance of an image correlation process, introduce scaling errors in computed movement measurements, and introduce angular anisotropy in the computed measurements. A better way of sensing movement may be desirable.